MOX (Mixed Oxide) fuel is at present produced industrially using a process known as MIMAS (MIcronized MASter blend). This process comprises successively a step of grinding oxides of uranium and plutonium, a step of diluting the powders obtained (primary blend) in the uranium oxide (UO2) and a step of sintering.
The MOX fuel produced by this process has a two-phase structure, one phase (U,Pu)O2 composed of solid solutions with a Pu/U+Pu content that can vary in the range of 30 to 5% of Pu and a UO2 phase. The (U,Pu)O2 phase exists either in aggregate form or in the form of “filaments” forming a continuous network in the fuel.
On irradiation, the fission gases are essentially created in the plutonium-bearing zones and install themselves over short distances (7 to 9 μm) then diffuse through the UO2 matrix before being released outside the fuel.
An improvement of the material consists in exacerbating the biphasic character of the present fuel to tend towards a material that isolates the plutonium-bearing zones in a UO2 matrix, which is intended to act as a retention barrier to the fission gases.